docs: def _output_dir(
          self,
          ext,
          is_instance=False,
          interpolatable=False,
          autohinted=False,
          is_variable=False,
      ):
          """Generate an output directory.
  
              Args:
                  ext: extension string.
                  is_instance: The output is instance font or not.
                  interpolatable: The output is interpolatable or not.
                  autohinted: The output is autohinted or not.
                  is_variable: The output is variable font or not.
              Return:
                  output directory string.
          """
  
          assert not (is_variable and any([is_instance, interpolatable]))
          # FIXME? Use user configurable destination folders.
          if is_variable:
              dir_prefix = "variable_"
          elif is_instance:
              dir_prefix = "instance_"
          else:
              dir_prefix = "master_"
          dir_suffix = "_interpolatable" if interpolatable else ""
          output_dir = dir_prefix + ext + dir_suffix
          if autohinted:
              output_dir = os.path.join("autohinted", output_dir)
          return output_dir